How does blender really save a blendfile?
According to the Blender wiki provided by lemon:

Saving and loading complex scenes in Blender is done within seconds. Blender achieves this by saving data in memory to disk without any transformations or translations. Blender only adds file-block-headers to this data.
A file-block-header contains clues on how to interpret the data. After the data, all internally Blender structures are stored. These structures act as blue-prints when Blender loads the file.

If I read this correctly, it means that blender just performs something like a memory dump and writes that to a file and just adds some meta data to it. This is a quick way to write big chunks of data to the harddrive, but makes for a though job when re-loading the .blend file.
This is because during load you must check for changes in platform, blender version,... as described in the wiki article as well.
All in all I think the answer to you question is yes: Blender does indeed write the full 1GB file to your harddrive. It is a lot faster than writing (for instance) a 1GB Word document because blender just 'dumps' the data rather than exporting it in a document format that is fairly easy to interpret on reload.